This invention relates to a discharge lamp with a discharge tube fixture arrangement. The invention further relates to a method for manufacturing a discharge lamp utilising such a fixture arrangement.
A wide variety of low pressure discharge lamps are known in the art. A large portion of the low pressure lamps on the market are so-called compact lamps, which means that they are intended to replace traditional incandescent bulbs. For this reason, compact lamps tend to have a discharge tube arrangement with approximately similar dimensions as those of traditional incandescent lamps. In many instances, the electric contacts of compact lamps are provided with traditional screw-in sockets, which form part of a lamp base. The lamp base normally houses the control gear (electronic ballast) of the discharge tubes, and provides a mechanical support for the discharge tube of the compact lamp. In a common discharge tube arrangement, the discharge tube is mechanically fastened to a cap of the lamp base, with the ends of the discharge tube being inserted into openings on the cap, also termed as a base cap.
In order to achieve a proper fastening of the discharge tube to the lamp base, various schemes are known. Older solutions proposed the use of cement, which is a traditional technology for fastening glass bulbs to the lamp base. This cement is in some cases based on artificial resin, which swells when baked and binds the glass body and the plastic cap to each other by creating a tensioning force between a part of the cap suitably designed and the wall of the glass body. In other cases this material is some kind of glue, which sets when affected by humidity or heat, and binds the glass body to the base cap, which is mostly made of plastic. The lamp's mechanical strength must satisfy standards and requirements of reliability and safety, and customer expectations during the whole lifetime of the lamp, and even beyond its lifetime. The lamp should be designed to prevent accidents even during a replacement. With the reduction in size of the compact lamps it is more and more difficult to use artificial resin based cement, since there is not enough space between the edges of the glass tubes and the internal wall of the cap for the cement to be placed, heated to a specified temperature and swell. More recently, the use of flexible adhesive (also commonly termed as glue) became known. Such an adhesive is applied to the ends of the discharge tube on the internal side of the base cap, and provides a mechanical connection between the discharge tube and the base cap. If glue is used, then only those glue types can be considered that are resistant to relatively high temperatures and UV radiation. Glue types that satisfy these requirements are the silicon based, thus flexible glue types. A glass body fixed with flexible glue possesses the necessary strength against mechanical stresses like drawing, twisting, bending, but cannot be used with the known plastic caps, since these do not provide the adequate surface size and shape necessary for bonding. For example, U.S. Pat. No. 5,697,697 teaches the use of either cement or adhesive. The base cap of the lamp base is provided with internal ribs, which latter define an adhesive space for receiving an amount of adhesive. The base cap or base plate is suitable for receiving either cement or adhesive.
This known fixture arrangement is suitable when the discharge tube is inserted relatively deeply into the lamp base, so that the ends of the discharge tube prortudes well below the level of the base plate. On the other hand, some problems remain. For example, the placing the glue on the inside of the base plate largely prevents the pulling out of the discharge tube, but provides insufficient protection against forces which push the discharge tube towards the inside of the lamp base.
A similar fixture arrangement is described in U.S. Pat. No. 5,772,310. In this known arrangement the base plate is equipped with a pair of ribs which guide the adhesive towards those openings which receive the ends of the discharge tubes.
Though the above solutions make it possible to attach a discharge tube mechanically to the base cap, other problems remain. Particularly, it is problematic to provide a suitable adhesive joint between the base cap and the discharge tube when the discharge tube does not extend very deeply into the openings of the base cap. This is generally the case with small compact lamps where the discharge tube is covered with an additional envelope, mostly for aesthetic reasons. In this case it is desirable to lift the discharge tube from the lamp base as much as possible, so that no or minimal light is shaded by the lamp base. Because the additional envelope is generally opaque, it does not matter that the discharge tube ends are not covered completely by the base plate. It is often more important to keep the volume of the lamp base close to size of those of traditional incandescent lamps, which again dictates that the discharge tubes should be lifted from the lamp base to the maximum possible extent.
Therefore, there is a need for a lamp with a fixture arrangement and a manufacturing method which provides a satisfactory mechanical joint between the ends of the discharge tube, by providing sufficient resistance against a pushing force acting on the discharge tube, and directed towards the inside of the lamp base. It is sought to provide an arrangement and method which, beside offering the reqired mechanical support, is relatively simple and which does not require expensive components and complicated manufacturing facilities, and which may be integrated into various types of existing production lines in a straightforward manner.